The present invention relates to plastic container closures, such as bottle cap liners and tamper evident seals, formed from a coextruded multilayer foamed film, to coextruded multilayer foamed films which are useful for plastic container closures, and to a method for coextruding such multilayer foamed films.
In a number of industries tamper evident seals are applied over plastic container openings for security purposes. Because of the container contents some of those industries also require that the seal keep liquid contents from leaking from the container, and/or keep air and contaminants from invading the container, and yet be easily removable. Industries having these requirements include the milk, orange juice, and motor oil industry. Common paper, foil, and paper-foil seals are often inadequate for use in those industries since they do not afford the quality of seal desired and since they are not easily applied by induction or high frequency sealing equipment.
Likewise, such seals are often inadequate when used as liners for threaded and snap-on bottle caps. In those instances a greater degree of compression is needed in the liner than is commonly found in paper, foil, and paper-foil seals.
Accordingly, other forms of plastic container closures have been developed. For example, it is known to use a single layer of a 5-10 mil thick closed-cell high density polyethylene (HDPE) foam film. However, such single foam layers are fragile, do not possess sufficient sealing properties for many uses and must be laminated to a metal foil and/or polyester film prior to being applied to the plastic container. Even then a number of disadvantages remain.
It has recently been suggested that a layer of compressible polyolefin foam may be adhered to a solid polymeric film to produce a liner suitable for a bottle cap. Thus in U.S. Pat. No. 4,818,577, assigned to Minnesota Mining and Manufacturing Co., there is disclosed a bottle cap liner which has a layer of compressible polyolefin foam, a layer of adhesive, and a layer of polymeric film such as polyester, silicone, polytetrafluoroethylene, and polyimide film. While such a multilayer foam/film is an improvement over a single foam layer, problems exist in the lamination process and still, as mentioned above, a number of disadvantages remain with laminated materials.
Finally, it is also known to use coextruded multilayer foamed films as plastic container closures. Thus, Tri-Seal International, Inc. of Blauvelt, N.Y. has recently introduced its Tri-Seal F-828 liners which are a 20-60 mil thick three-ply coextruded foamed polypropylene core between two facings of solid polypropylene. The solid polypropylene film facings are said to protect the container contents from penetration and evaporation while the foamed plastic core is said to be resilient, compressible, and resistant to foam collapse. The Tri-Seal F-828 liners are also said to be capable of radially expanding under pressure should they be used with plastic or metal bottle caps. Still, because of their relative thickness, applicability is limited. U.S. Pat. Nos. 4,107,247 and 4,206,165 assigned to Tri-Seal International, are believed to cover the method of coextruding such multilayer foamed film liners.
Other methods for coextruding multilayer foamed films are also known. For example, U.S. Pat. No. 3,557,265, assigned to the assignee of the present invention, discloses a method of extruding laminates whereby optionally alternating foamed and unfoamed layers may be obtained. See also U.S. Pat. Nos. 4,022,858 and 3,553,070, assigned to Mobil. More recently, U.S. Pat. Nos. 4,533,578 and 4,657,811 to Mobil disclose coextrusion methods for producing a relatively thin three-layer polyolefin film having a foamed middle layer. The coextruded multilayer foamed film is adapted for use in high performance polyolefin trash bags. The outer facing layers are linear low density polyethylene (LLDPE) or high density polyethylene (HDPE) and the foam core is a low density polyethylene (LDPE). Finally Published German Patent Application DE No. 3722139 dated Jan. 19, 1989, assigned to Stoll Kunststoffe, discloses producing a thermoplastic foamed film particularly for carrier bags and packaging bags by coextruding at least two layers (which may be polyolefin), one containing a blowing agent and one containing no blowing agent. The final thickness of the individual layers is 6-18, and preferably about 12, microns. There is no suggestion in these later patents that such foamed films could be used as plastic container closures, nor as a practical matter could they be effectively so used.
Thus while considerable technology exists in the foamed film area, to date it has not yet been possible to produce a plastic container closure which satisfies all of the needs of the industries which package their contents in plastic containers (bottles, jars, and jugs). In particular, the need exists for a relatively thin (less than about 10 mils total thickness) multilayer foamed film which has at least one solid layer for overall strength and barrier resistance and a foamed layer which is as strong as possible while still having the desired degree of resilience and compressibility such that it can be used for a plastic container closure. The need also exists for an effective method of producing such multilayer foamed films.